146
FXUS66 KPDT 221713
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
913 AM PST Fri Nov 22 2024

.MORNING UPDATE...Main update to the forecast this morning was to
include mention of patchy dense fog through morning for portions
of central and north-central Oregon as well as along the foothills
of the Blue Mountains of OR/WA. Have issued a Special Weather
Statement for the fog, but areal coverage does not appear to be
enough to warrant a dense fog advisory for any forecast zones.
Plunkett/86

&&

.PREV DISCUSSION... /issued 235 AM PST Fri Nov 22 2024/

SHORT TERM...Today through Sunday...A closed upper level low
pressure system continues off the coast with several shortwaves
rotating around it. One is currently moving north off the coast
and lifting a warm moist airmass across the forecast area in the
from of a warm front. Snow levels will be 5000 to 7000 feet
through the morning meaning rain in most of the forecast area and
snow only in the highest elevations. The warm front will lift
north of the forecast area by midday pushing most of the rain
north of the forecast area. Then a cold front will begin to push
west to east across the forecast area this afternoon and overnight
accompanied by a slow moving band of precipitation that moves off
into Idaho on Saturday. With it we will see a lowering of snow
levels 3000 to 4500 feet by Saturday morning with a return of some
snow in the higher terrain and mountains. The low pressure system
remains off the coast on Saturday which keeps a moist southwest
flow across the forecast area and mainly some mountain showers.
Overall, snow amounts look to remain below advisory levels for the
mountains during this event but passes will see some snow
accumulations.

The low pressure consolidates into a smaller low off the coast
Saturday night through Sunday while still keeping the forecast
area under a southwest flow and with snow levels 2500 to 3500
feet. This will lead to some continued showers over some of the
forecast area but with lite precipitation amounts.

Winds will continue to be an issue along the base of the Blue
Mountain Foothills and the southern Grande Ronde Valley until the
cold front passes through the area this afternoon. The wind
advisories will continue for these areas at least until the late
afternoon.

LONG TERM...Monday through Friday...A colder and drier pattern is
in store for the forecast area for Thanksgiving week, as ensembles
broadly favor a transition from broad troughing towards that of
dry northerly flow by the end of next week. Best chances for
precip would be across the high mountains, but even with snow
levels lowering, QPF across ensemble guidance is unimpressive,
reflecting the drier air mass moving in under the expected
synoptic pattern change.

Not much disagreement to start off the period as ensemble
consensus points toward the broad, offshore low that has been
plaguing the forecast area with windy and wet conditions for the
past week finally moving onshore by late Monday. Colder air starts
to move in as a result, and the instability provided by the low`s
center will support broad PoPs (30-40% valleys, 60-70% mountains)
across the area, with snow levels around 3000-4000 feet, but with
meager QPF as this low lacks a sufficient moisture advection
mechanism. This low moves out of the PacNW by late Tuesday,
leaving us with dry and cold northerly flow in its wake. High
temps will struggle to climb out of the mid 40s, and we certainly
can`t rule out pockets of Basin fog, given the relatively
saturated airmass and benign wind pattern, however confidence is
too low (<20%) to mention the possibility directly in the
forecast.

PoPs gradually scale back for the remainder of the period. A
slight chance (15-25%) of PoPs persists across the high
mountains in the wake of upslope, northerly flow, but as of now,
confidence is moderate to high (60-70%) that the area will not see
a significant precip event that would otherwise impact
Thanksgiving travels, at least through Wednesday and Thursday.
Models split by the tail end of the period, with the EPS broadly
favoring a persistent north to northwesterly flow solution,
perhaps with an embedded shortwave, while the GEFS suggests
ridging may move in after Thanksgiving. Forecast confidence beyond
Thanksgiving day is low, however (<30%). Evans/74

AVIATION...12z TAFs...Variable conditions expected this morning as
low cigs and fog impact many sites in the Basin, complicated by
oncoming rain and wind expected through the late morning.
Expectation is that fog will lift as rain moves in, with sites such
as PDT and ALW still seeing IFR or worse conditions due to low
cigs,  with all sites seeing VFR by the late afternoon/early
evening as conditions dry and winds in PDT/ALW mix out fog.
Another round of rain is then expected around nightfall Friday
night, impacting most sites, save DLS and YKM, with at least MVFR
conditions. Evans/74

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  59  40  48  34 /  80  90  60  20
ALW  58  40  49  36 /  90 100  80  20
PSC  51  42  51  38 / 100  80  30  20
YKM  47  31  47  31 / 100  20  10  20
HRI  53  41  51  35 / 100  90  40  20
ELN  46  30  45  30 / 100  20  10  20
RDM  55  34  44  30 /  60  50  20  10
LGD  48  37  44  29 / 100 100  90  20
GCD  54  37  44  25 /  80 100  80  20
DLS  53  40  52  37 / 100  30  20  30

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 3 PM PST this afternoon for ORZ049-507.

WA...Wind Advisory until 3 PM PST this afternoon for WAZ029.

&&

$$

MORNING UPDATE...86
SHORT TERM...91
LONG TERM....74
AVIATION...74